A Rouen is like what would happen if a mallard landed in New Zealand before there were stoats, cats, ferret/polecat hybrids. The selection pressures that keep birds flighted must have to be in force constantly because there are tons of flightless birds on islands, and just about every domestic duck or goose strain can’t fly or can’t fly very far. Even pheasants that bred for shooting purposes have to be selected constantly for flying or they become flightless.